1. Field of the Invention
The present invention relates to an electrophotographic image formation method based on the Carlson process to obtain visible toner images on an image-receiving medium using a function-separating laminated-type electrophotographic photoconductor comprising an electroconductive support, and a photoconductive layer comprising a charge generation layer and a charge transport layer which are successively formed on the support.
2. Discussion of Background
According to the Carlson process, the surface of an electrophotographic photoconductor is uniformly charged to a predetermined polarity and the charged surface is exposed to light images to form electrostatic latent images thereon, and then the electrostatic images thus formed are developed to visible toner images by a developer. In addition to the above, the toner images are transferred to an image-receiving medium such as a sheet of paper and fixed thereon.
In the electrophotographic photoconductor, inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide are conventionally used. Recently, photoconductors comprising organic photoconductive materials such as poly-N-vinylcarbazole and polyvinyl anthracene have been studied and developed.
The electrophotographic photoconductors employing organic photoconductive materials are divided into two groups. One is a single-layered photoconductor comprising an electroconductive support and a photoconductive layer formed on the support, in which a charge generating material and a charge transporting material are dispersed in a binder resin; and the other is a function-separating laminated-type photoconductor comprising an electroconductive support, and a charge generation layer and a charge transport layer which are successively overlaid on the support. The charge generation layer and the charge transport layer respectively comprise a charge generating material and a charge transporting material, each of which is dispersed in a binder resin in the layer. The characteristics of the organic photoconductive materials are drastically improved when they are used in the above-mentioned function-separating laminated-type photoconductor because appropriate materials constituting the charge generation layer and the charge transport layer can be individually selected.
However, the laminated-type electrophotographic photoconductor has the shortcoming that, for instance, when the charge transport layer is formed on the charge generation layer, the charge transport layer is easily worn when coming in contact with various members such as a developer at a development step, an image-receiving medium at an image-transfer step, and a cleaning member at a cleaning step because the charge transport layer comprises organic materials. Due to the wear of the charge transport layer, the photosensitivity of the photoconductor is decreased, with the result that abnormal images such as toner deposition on the background are induced.
To improve the wear-resistance and the durability of the charge transport layer, the following photoconductors are proposed:
(1) a photoconductor comprising a charge transport layer with a thickness of 25 .mu.m or more, as disclosed in Japanese Laid-Open Patent Application 1-267551; PA1 (2) a photoconductor comprising a protective layer formed on a charge transport layer, which comprises a binder resin comprising as the main component polyurethane, as disclosed in Japanese Laid-Open Patent Application 58-122553; PA1 (3) a photoconductor comprising a protective layer formed on a charge transport layer, which comprises a hardening silicone resin, as disclosed in Japanese Laid-Open Patent Application 61-51155; PA1 (4) a photoconductor comprising a protective layer formed on a charge transport layer, which comprises as the main component polyetherimide, as disclosed in Japanese Laid-Open Patent Application 2-161449; PA1 (5) a photoconductor comprising multiple charge transport layers, with the concentration of a charge transporting material in each charge transport layer being decreased toward the surface of the photoconductor, as disclosed in Japanese Laid-Open Patent Application 2-160247; and PA1 (6) a photoconductor comprising a surface layer formed on a charge transport layer, which comprises finely-divided, spherical particles of a resin such as a silicone resin, as disclosed in Japanese Laid-Open Patent Application 63-2072.
However, the above-mentioned photoconductors have their own drawbacks. For instance, the deterioration in photosensitivity of the photoconductor (1) due to the wear of the charge transport layer can be reduced to some extent, but the photosensitivity is not sufficient for use in practice. In addition, the deterioration performance of a coating liquid for the charge transport layer of this type is poor in the coating operation, so that the obtained charge transport layer becomes uneven. Therefore, it is necessary to improve the facilities for producing such a photoconductor, which leads to the rise of manufacturing cost.
The photoconductor (2) has the drawback that image blurring occurs under the atmosphere of high humidity because the surface resistivity of the photoconductor is decreased.
The residual potential of the photoconductor (3) is apt to increase, with the result that the toner deposition on the background occurs at a relatively early stage during a repeated copying operation.
The drawback of the photoconductor (4) is that the photosensitivity considerably deteriorates and the residual potential readily increases, which causes the toner deposition on the background.
In the photoconductor (5), since a lower charge transport layer is dissolved in a coating liquid for an upper charge transport layer while it is applied to the lower charge transport layer, the charge transporting material in the lower charge transport layer transfers to the upper charge transport layer. In fact, the ratio of the charge transporting material to the binder resin in the upper charge transport layer is increased as compared with that in the lower charge transport layer, so that it is difficult to improve the wear resistance of the charge transport layer.
Furthermore, to prevent the decrease of the photosensitivity of the photoconductor, several methods are known, for example, a method of controlling the exposure of a photoconductor in accordance with the detection of the total number of revolutions of the photoconductor and the total charging time thereof, as disclosed in Japanese Laid-Open Patent Application 4-26871; and a method of scraping a portion caused to deteriorate by ozone from a photoconductor while in use, as disclosed in Japanese Laid-Open Patent Application 1-133086.
However, the former method has the shortcoming that the process itself is so complex that an apparatus for executing this process becomes complicated. By the latter method, the deterioration in photosensitivity cannot be sufficiently prevented.